Electronic switching power supplies are used to convert alternating current (AC) to direct current (DC) to provide power to electrical circuits and electronic circuits in controls, communications, lighting and numerous other fields. Because switching power supplies operate at switching frequencies that are within regulated frequency bands, the switching power supplies must conform to regulations imposed by the Federal Communications Commission (FCC) to maintain electromagnetic interference (EMI) within acceptable levels. The FCC regulations of EMI generally are directed to conducted emissions and to radiated emissions.
In conventional electronic circuits, the reduction of conducted emissions and the reduction of radiated emissions are accomplished by different components. A typical common-mode choke comprises a first winding and a second winding wound onto a single common core. The common core may comprise two core bodies that are mated to provide a single core. For example, two E-cores may be installed in a single bobbin to provide a single EE core structure. In such a structure, one or more coils are wound around the abutted center legs of the two E-cores. The AC line current from the line connection of an AC source passes through one of the windings, and the AC neutral current from the neutral connection of the AC source passes through the other winding. The two windings are configured to produce a large inductance to common-mode currents passing through the two windings to reduce the common-mode currents and thereby reduce conducted emissions.
A common-mode choke does not effectively control radiated emissions. Thus, a typical electronic circuit with a switching power supply also includes a first RF inductor (bead) in the path between the AC line (hot) connection and the common-mode choke and a second RF inductor in the path between the AC neutral connection and the common-mode choke. A typical RF inductor has a very low inductance and has a very small parasitic winding capacitance. RF inductors are effective for reducing radiated high frequency RF noise in a range of approximately 30 MHz to approximately 300 MHz.
Although RF inductors are effective to reduce to reduce radiate RF noise, the RF inductors add extra cost to the electronic circuit into which the RF inductors are incorporated. Furthermore, the RF inductors require substantial areas on printed circuit boards.